What is Wilms tumour?

During pregnancy, the kidneys are some of the first organs to develop in a fetus. Cells called nephroblasts grow and divide quickly to create the kidneys. At a certain point, the nephroblasts stop growing and dividing and they mature into kidney cells. But sometimes nephroblasts continue to grow out of control instead of maturing into kidney cells. These abnormal nephroblasts may lead to non-cancerous (benign) tumours such as congenital mesoblastic nephroma, which develops in newborns and infants.

In some cases, the abnormal nephroblasts can cause precancerous changes called nephrogenic rests. Nephrogenic rests are groups of abnormal kidney cells that may grow in many areas of the kidney or make a thick layer around the kidney. When this happens, the precancerous condition is called nephroblastomatosis. Nephroblastomatosis usually occurs in both kidneys and may become cancer, called Wilms tumour, if left untreated.

Wilms tumour is the most common type of kidney cancer in children. Itʼs usually diagnosed in children 2 to 4 years old.

Most Wilms tumours are found in only one kidney (called unilateral Wilms tumour). In about 5% of cases, it is found in both kidneys (called bilateral Wilms tumour). It may be in both kidneys at the time of diagnosis, or it may develop in the other kidney a few years after being found in the first kidney. Bilateral Wilms tumour is more common in children who have an inherited condition that increases the risk of Wilms tumour, such as Beckwith-Wiedemann syndrome or WAGR syndrome.

There are 2 types of Wilms tumour, and each type is determined by how the cells look under a microscope (called histology). Doctors tell the 2 types apart based on whether anaplasia is present. Anaplasia is a loss of differentiation in the cells, which means they donʼt look or act like normal cells.

About 90% to 95% of Wilms tumours are classified as having a favourable histology, which means there is no anaplasia present.

Wilms tumours with a favourable histology respond better to chemotherapy, so they have a better prognosis (outcome) than tumours with anaplasia present (anaplastic histology).

About 5% to 10% of Wilms tumours are classified as having an anaplastic histology. An anaplastic histology may also be called an unfavourable histology. The cells in these tumours look very different from normal kidney cells and divide abnormally.

Anaplasia may be focal or diffuse.

Focal anaplasia means anaplasia is found only in one or a few specific areas of the tumour. The rest of the tumour looks like a regular (favourable histology) Wilms tumour.

Diffuse anaplasia means anaplasia is spread throughout the tumour.

Anaplastic tumours are more difficult to treat because they tend to be more resistant to chemotherapy. Anaplastic tumours are more likely to recur (come back) after treatment than tumours with a favourable histology.